The synchronous continuous casting process is a synchronous continuous casting process that gives rise to no difference in the relative velocity between a cast strip and an inner wall of a mold, such as a twin-roll system, a twin-belt system or a single roll process, as introduced, for example, in feature articles in "TETSU TO HAGANE (Journal of The Iron and Steel Institute of Japan)", 85-A197 to A256. The twin-roll continuous casting process, which is a synchronous continuous casting process, is a continuous casting process wherein a molten steel is poured into a continuous casting mold comprising a pair of cooling rolls having the same or different diameter and disposed parallel to each other or disposed so as to have an inclined positional relationship and side weirs for sealing both end faces of the cooling rolls to provide solidified shells on the respective circumferential surfaces of both the cooling rolls, which solidified shells are united with each other around the nearest approach point between the two cooling rolls being rotated (the so-called "kissing point") to deliver the solidified shells as an integral thin cast strip.
The thin cast strip provided by the twin-roll continuous casting process has a thickness of several mm (usually in the range of from about 1 to 10 mm) and can be cold-rolled without prior hot rolling to provide a thin sheet product. Therefore, the twin-roll continuous casting process has marked advantages in the production efficiency and cost over a production process (slab casting-hot rolling process) which comprises casting a slab having a thickness exceeding 100 mm by continuous casting using a vibratory mold or the like, hot-rolling the slab and cold-rolling the hot-rolled sheet.
The thin sheet of an austenitic stainless steel produced by cold rolling is subjected to forming, such as bending, burring, drawing and stretching, and widely used in applications, such as building materials, dinnerware and kitchen fitments. Therefore, the sheet should have an excellent appearance after forming, not to mention excellent formability. The materials provided by the conventional hot rolling process had the product characteristics required in the above-described applications. On the other hand, the materials produced by the twin-roll continuous casting process gave rise to the following new problems when used in some of the above-described applications.
Specifically, as a result of various studies, the present inventors have found that products produced by subjecting a thin cast strip provided by the twin-roll continuous casting process or other process to cold rolling without hot rolling suffer from surface roughening along the rolling direction when subjected to forming (particularly drawing or stretching). As opposed to the orange peel phenomenon known in the art, which depends upon the grain size of the cold-rolled product sheet, the surface roughening occurs in the form of small wavy surface roughening having an average size of about several mm or less in length and 0.5 mm or less in width (hereinafter referred to simply as "type A surface roughening") and large flow pattern surface roughening having an average size of several hundred mm or less in length and about 3 mm or less in width (hereinafter referred to simply as "type B surface roughening") either alone or in combination. This surface roughening is liable to occur particularly in the stretching of BA products (bright-annealed products), which remarkably spoils the appearance of the formed article.
The surface roughening phenomenon caused in the forming is different also from "roping" that is a surface roughening phenomenon caused in the cold rolling, so that it is necessary to take new measures to prevent the type A surface roughening and type B surface roughening. For example, Japanese Unexamined Patent Publication (Kokai) Nos. 2-13352 and 2-133522 specify the average .gamma. grain size of the cast strip for the purpose of preventing roping. Further, Japanese Unexamined Patent Publication (Kokai) No. 2-19426 describes the refinement of the average .gamma. grain size by recrystallization through intermediate annealing. These methods, however, cannot completely prevent the occurrence of surface roughening in the forming of a cold-rolled product. The surface roughening in thin sheet products has not been recognized in the above-described prior art, so that no measures have been suggested.